This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-250299, filed Sep. 3, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a ball screw apparatus used for a feed mechanism or the like of various devices.
A ball screw apparatus is a machine element for converting a rotary motion into a linear motion for example. The ball screw apparatus comprises a screw shaft on whose outer peripheral surface a first helical groove for rolling balls is formed, and a nut to be fitted on an inner periphery of the screw shaft. A second helical groove associated with the first helical groove is formed on an inner peripheral surface of the nut.
A helical rolling contact way of balls is formed by the first and second helical grooves facing each other. A part and another part of the rolling contact way, which are apart from each other, communicate with each other via a connecting way provided on the nut.
An endless ball circuit is defined by the rolling contact way of balls and the connecting way. A number of balls are contained in the ball circuit. The material of the balls is steel, ceramic, plastic, or resin, etc. These balls are endlessly and successively arranged within the ball circuit.
When the screw shaft rotates relative to the nut, the rotation is transmitted to the nut via the balls between the screw shaft and the nut, and thereby the nut moves in the axial direction of the screw shaft. In company with this movement, each ball endlessly circulates in the ball circuit. By this endless circulation of the balls, the nut moves smoothly with high accuracy in the axial direction of the screw shaft in accordance with the rotation of the screw shaft.
FIG. 6 shows a typical view of a cross section of a conventional tube-type ball screw apparatus 100. A nut 2 is mounted on an outer periphery of a screw shaft 1. A first helical groove 1a is formed on an outer peripheral surface of the screw shaft 1. A second helical groove 2a facing the first helical groove 1a is formed on an inner peripheral surface of the nut 2. A helical rolling contact way of balls 3 is formed by the first and second helical grooves 1a and 2a. 
Through holes 4a and 4b are formed at a peripheral wall of the nut 2. These through holes 4a and 4b are connected to each other by a tube 5 provided on an outer peripheral part of the nut 2. Both ends of the tube 5 are inserted in the through holes 4a and 4b respectively. The through holes 4a and 4b and the tube 5 constitute a connecting way 6 for communicating a part and another part of the rolling contact way 3, which are apart from each other.
A ball circuit 7 is defined by the rolling contact way 3 communicating endlessly via the connecting way 6. A number of balls 8 are contained in the ball circuit 7 so as to succeed endlessly. These balls 8 circulate in the ball circuit 7 while rolling in the ball circuit 7 when the screw shaft 1 and the nut 2 relatively rotate.
In this ball screw apparatus 100, for example, when the nut 2 rotates with respect to the screw shaft 1 in a direction shown by an arrow R in FIG. 7, each ball 8 rolls along the helical groove 1a in the direction of the arrow R. Since the rotation speed of the nut 2 is faster than the moving speed of the balls 8, each ball 8 moves from the tube 5 toward the helical groove 2a of the nut 2 via one through hole 4a. 
In the conventional ball screw apparatus 100, for a while after starting use of the apparatus, as shown in FIG. 7, there is an angular portion 10 between the rolling contact way 3 and the connecting way 6, specifically, in the boundary between the helical groove 2a of the nut 2 and the connecting way 6. There is a problem that this angular portion 10 prevents smooth movement of balls 8 circulating through the ball circuit 7, and operability and durability of the apparatus deteriorates.
In the meantime, when an external load acts on the ball screw apparatus 100, the distance between the helical grooves 1a and 2a becomes narrow by Hertz contact between the first helical groove 1a and-balls 8, Hertz contact between the second helical groove 2a and balls 8, and elastic deformation of the balls 8 themselves. This state is referred to as xe2x80x9celastic narrownessxe2x80x9d in this specification.
In the state where there is elastic narrowness, the balls 8 enters from the tube 5 which is a non-loading region to the part between the helical grooves 1a and 2a which is a loading region. When the elastic narrowness exists in the helical grooves 1a and 2a, balls 8 cannot enter between the helical grooves 1a and 2a in this state. Therefore, a preceding ball 8 enters between the helical grooves 1a and 2a by being pushed by a following ball 8.
In the conventional art, the helical groove 2a of the nut 2 is machined continuously through the whole inner periphery of the nut 2, on the basis of the center diameter of the balls 8. Therefore, when the external load acts, the helical grooves 1a and 2a come into the state of elastic narrowness equally through the whole inner periphery of the nut 2. Consequently, at the instant when the ball 8 enters from the tube 5 to the helical grooves 1a and 2a, the ball 8 is rapidly compressed.
For example, in a ball screw apparatus having a diameter of 100 mm, lead of 25 mm, and a ball diameter of 19.05 mm, when the apparatus is loaded, sometimes elastic narrowness quantity between the helical grooves reaches 50 xcexcm. In this case, when the ball enters the loading region from the non-loading region, the ball is compressed by almost 50 xcexcm. Therefore, in particular when the load is large, there is the fear of causing the following problems:
(1) stress is concentrated on the angular portion 10 between the helical groove 2a and the end of the tube 5, and flaking occurs from the part as the origin;
(2) the balls 8 are liable to stop in the vicinity of the angular portion 10, and the balls 8 cannot smoothly circulate; and
(3) when the ball 8 enters the helical groove 2a from the tube 5, the preceding ball 8 is pushed by the following ball 8, and thereby the ball is damaged and worn. Particles generated by wearing deteriorate durability of the ball screw apparatus.
Therefore, the object of the present invention is to provide a ball screw apparatus which can circulate balls more smoothly to maintain good operability and improve durability.
A ball screw apparatus of the present invention for achieving the above object comprises:
a screw shaft having a first helical groove at an outer peripheral surface thereof;
a nut fitted on an outer periphery of the screw shaft, the nut having at an inner peripheral surface thereof a second helical groove which is associated with the first helical groove;
a rolling contact way defined by the first and second helical grooves facing each other;
a connecting way provided on the nut, the connecting way communicating a part and another part of the rolling contact way;
a ball circuit defined by the connecting way and the rolling contact way in an endlessly communicating fashion; and
a plurality of balls contained in the ball circuit such that the balls are endlessly arranged, and
a machined portion with a smooth shape which is formed on the inner peripheral surface of the nut by cutting out an angular portion between the rolling contact way and the connecting way.
The machined portion formed by means of crowning or the like has such a shape that a distance between the helical grooves gradually decreases in a direction in which balls enter between the helical grooves from the connecting way. The machined portion preferably extends from an end of the connecting way in a circumferential direction of the nut and is formed within a range of 90xc2x0 from the angular portion in the circumferential direction of the nut. Thereby, when a ball enters the rolling contact way from the connecting way, the ball is gradually compressed, and concentration of stress on the helical groove can be avoided. If the machined portion exceeds the range of 90xc2x0 from the angular portion in the circumferential direction of the nut, the load capacity of the ball screw apparatus is reduced. Therefore, the maximum region for forming the machined portion is 90xc2x0 from the angular portion.
The cutting depth of the machined portion is preferably greater than the elastic narrowness amount of the helical groove. In particular, the range of the cutting depth in the radial direction of the nut is preferably from {fraction (1/400)} to {fraction (1/10)} of the diameter of the ball. If the cutting depth is equal to or less than {fraction (1/400)} of the diameter of the ball, the balls do not move smoothly, and the above object to be solved by the present invention cannot be solved. If the cutting depth exceeds {fraction (1/10)} of the diameter of the ball, the operability of the ball screw apparatus deteriorates.
According to the present invention as described above, by forming the machined portion between the rolling contact way and the connecting way, the balls circulate smoothly from the beginning of manufacturing the ball screw apparatus. The ball screw apparatus of the present invention has a good operability, and the durability of the apparatus can be enhanced. Further, since the balls can easily enter the loading region between the helical grooves from the non-loading region inside the connecting way, damage and wearing due to contact of the balls can be restrained.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.